The traditional technique for energizing fluorescent lamps employs a relatively inefficient autotransformer operable at a 60-Hz rate. The transformer is purposefully designed to be inefficient in order to act as a current limiter or ballast circuit whereby excessive current flow is prevented. Obviously, such energy waste and excessive heat generation is undesirable and heavy and expensive autotransformers leave much to be desired.
Recent efforts have focused upon the design of solid state ballasts for driving fluorescent lamps at relatively high frequencies, such as about 20 KHz for example. Such circuitry not only tends to reduce the weight and size of the ballasting operation but also increases the efficiency and predictability of performance while virtually eliminating noise within the audible range.
However, the known ballast circuitry suitable for solid state designs is, for the most part, of a multiple semiconductor design and includes multiple power transistor elements. Unfortunately, multiple power transistor elements present difficulties when integrated designs are attempted due to the multiple potentials required by the individual semiconductor elements. In other words, supplying a multitude of different voltages to semiconductors on a single integrated substrate presents numerous problems in manufacture, cost, efficiency and performance.
Additionally, the known circuitry employing a single transistor does not provide an output potential suitable for use with fluorescent lamps. More specifically, the known forms of single power transistor oscillator circuitry generate an asymmetric output potential including a DC component. Unfortunately, it is not desirable to operate fluorescent lamps with a potential that includes a DC component due to the tendency toward so-called "mercury migration" wherein a linear fluorescent lamp tends to exhibit an undesired differential in light output from one end to the other when the operational potential includes a DC component.